1. Field of the Invention
The present invention relates to a wireless AC power feeding and, more particularly, to power control thereof.
2. Description of Related Art
As an eco-friendly next-generation vehicle, an EV (Electric Vehicle) is now attracting attention. Since the EV travels by an electric motor, it needs to be provided with a secondary battery (storage battery). It is said, in general, that a travel distance of as small as about 100 km is achieved by a fully charged secondary battery mounted in the EV. When the capacity of the second battery is increased, the vehicle weight is unfavorably increased.
A wireless power feeding technique of feeding power without a power cord is now attracting attention. The current wireless power feeding technique is roughly divided into three: (A) type utilizing electromagnetic induction (for short range); (B) type utilizing radio wave (for long range); and (C) type utilizing resonance phenomenon of magnetic field (for intermediate range).
Among the above types (A) to (C), the type (C) utilizing magnetic field resonance phenomenon is of particular interest because of its high power transmission efficiency. For example, a plan is being studied in which a receiving coil is buried in a lower portion of the EV so as to feed power from a feeding coil in the ground in a non-contact manner (Refer to Patent Document 7). The wireless configuration allows a completely insulated system to be achieved, which is especially effective for power feeding in the rain. Hereinafter, the type (C) is referred to as “magnetic field resonance type”.
The magnetic field resonance type is based on a theory published by Massachusetts Institute of Technology in 2006 (refer to Patent Document 1). In the magnetic resonance type, a resonance circuit (LC circuit) is formed on both the power feeding side and power receiving side, respectively. The resonance frequency of the power feeding side resonance circuit and that of the power receiving side resonance circuit are made to coincide with each other. When the power feeding side resonance circuit is made to resonate at a resonance frequency fr1, the power receiving side resonance circuit resonates at a resonance frequency fr1. At this time, AC power can be fed with the maximum power transmission efficiency (refer to Patent Document 6).